A number of processes require the measurement and control of the movement of flowable solids particles.
Many processes are known in which the starting material or the product consists of solids which may be present in the form of a stream of solid particles of approximately the same structure, the same composition and/or shape, or of a mixture of such particles. More specifically, such materials may be defined, for example, as lumps, broken lumps, stone chips, grains, compacts, pellets, pills, extrudates, salt-like substances, dusts, powders, etc.
The fluidized-bed technology involves starting materials, products, and/or catalysts in the form of fine-grain solid materials. The fluidized-bed drying of bulk materials or the gassification of pulverized coal with oxygen by fluidized-bed method (Winkler process) may be cited in this connection. Another example is the fluidized-bed firing in thermal power stations where pulverized coal is burned in the presence of lime-bearing granular materials. Fluid-bed catalytic processes are the state of the art in petroleum refining technology.
All these processes involve the handling of streams of flowable solid materials, wherein the streams require measurement and control. The material is frequently subject to pressure. The pressure may be the static pressure of a column of flowable solids above the measuring point, or the dynamic pressure of a flowing stream of solids which may be subject to transformation into a static pressure at points of restriction, or the pressure of a liquid or gas entrained in the flowing stream of solid materials. The flowing stream to be measured is sometimes at an elevated temperature, for example, several hundred degrees centigrade, which requires an enclosure to avoid heat dissipation.
In many cases, there are other reasons to prevent any contact of material and/or the entrained liquid or gas with the atmosphere or with any other ambient substance. Certain solids are pyrophorous when being contacted with air at ambient temperature. The entrained gas may be inflammable or toxic.
A number of methods are known for measuring such flowing streams of solids in closed systems.
The streams of solids may be measured by means of a liquid-embedded gate which is opened by the force of the flowing material against the force of a counter weight or spring. This method is subject to substantial inaccuracies which depend, among other factors, on the grain size distribution of the granular or lumpy material.
Bucket wheels are suitable for feeding and metering but do not lend themselves to measuring flowing streams of solids. The same applies to pairs of gear wheels (reversible of the gear pump principle). Flowability of the solids is generally insufficient to impart rotary motion to the wheels.
For the counting and sorting of solids unit quantities of the same shape, for example, pills, pellets, compacts, use is made of horizontal discs which are provided with circumferential cavities for accommodating one unit quantity each. These devices have the disadvantage that different shapes and sizes of the unit quantities are bound to cause difficulties and that the unit quantities must have a certain definite shape.
Measurement is frequently performed with the aid of a set of locks. This system comprises one or several receivers connected in parallel with their associated upstream and downstream shut-off devices. One receiver is filled to a certain definite level with the outlet in the closed position. It is then emptied while the inlet is closed. Several receivers connected in parallel may be operated intermittently. The receiver may rest on rubber supports or springs and may be connected through expansion joints to the other structural elements of the system. The weight of the processed material can be measured directly through the deformation of the rubber supports or springs.
The lock system permits measuring of the volume and/or weight of the solids in the flowing stream under pressure and exclusion of air. Intermittent operation is a disadvantage of this system. In addition, it requires voluminous equipment comprising vessels, valves, interconnecting piping, distributors, control facilities, etc. For certain streams of solids, it involves the risk of the material being inadmissibly crushed between elements of the shut-off devices. The use of expansion joints limits the pressure and temperature levels.